Palaeoecology of late middle Eocene planktic foraminifera and evolutionary implications

Abstract

The late middle Eocene is marked by accelerated global cooling, representing part of the long transitional interval separating the early Eocene ‘greenhouse’ and later Oligocene ‘icehouse’ climatic regimes. This time interval is also regarded as having witnessed a decline in planktic foraminiferal diversity following the extinction of most of the muricate clade (genera Acarinina and Morozovelloides) at approximately 39 Ma. Here we examine planktic foraminiferal assemblages recovered at ODP Site 1052 (Blake Nose, NW Atlantic) and, by recording a high level of taxonomic diversity including several previously undocumented morphotypes, we suggest that the diversity of late middle Eocene planktic foraminifera may have been underestimated. Depth habitats of every species within these late middle Eocene assemblages are reconstructed using oxygen and carbon stable isotope ratios. Furthermore, because stable isotope analyses suggest that species of Globoturborotalita appear to have generally calcified during winter months, these taxa offer, in combination with predominantly summer surface dwellers such as Morozovelloides, the potential to reconstruct patterns of seasonality during the late Palaeogene. Using extremely well preserved ‘glassy’ planktic foraminiferal calcite from a contemporaneous hemipelagic drill site, detailed SEM imaging of test microstructures and wall textures confirm that many modern features of foraminiferal ecology and test architecture had already evolved by the late middle Eocene. The common occurrence of the cancellate wall texture, combined with the wide variety of reconstructed foraminiferal depth habitats, indicates that there is no necessary relationship between foraminiferal wall textures and habitat and that wall textures are evolutionary conservative features of the foraminiferal test. <br/